vSAN for Rackspace Private Cloud powered by VMware (RPC-VMware)
contains the following components.

Component

Description

Host

ESXi hosts (hypervisors) run the vSAN software and provide the local
drives that are used for the cache and capacity tiers.

Disk group

A disk group is a unit of physical storage capacity on a host and a
group of physical devices that provide performance and capacity to
the vSAN cluster. On each ESXi host that contributes its
local devices to a vSAN cluster, devices are organized into
disk groups.

Capacity tier

The capacity tier is used to permanently store data that is stored
in vSAN. Cold data not in the cache tier is written to the capacity
tier. The capacity tier is made up of flash disks.

Cache tier or flash cache

Each disk group must have one flash cache device and one or more
capacity devices. The devices used for caching cannot be shared
across disk groups, and cannot be used for other purposes. A single
caching device must be dedicated to a single disk group.

RPC-VMware uses only all-flash clusters. In an
all-flash cluster, flash devices are used for both the cache tier and
capacity tier. In all-flash clusters, the cache tier is used to cache
writes only.

vSAN datastore

The datastore that is provided by vSAN and backed by the aggregate
storage of all drives claimed by vSAN.

VM storage policy

VM-level availability and performance policies that can be set and
modified as needed. vSAN works with VM storage policies to support a
VM-centric storage approach. If no storage policy is explicitly assigned
to a VM is when it is provisioned, a generic system-defined storage
policy called the vSAN Default Storage Policy is automatically applied
to the VM.

Deduplication and compression

Deduplication and compression occur when data is destaged from the
cache tier to the capacity tier. This function is set when you
enable “space efficiency” on a cluster level. Deduplication happens
on a per disk group basis. Bigger disk groups result in a higher
deduplication ratio. After the blocks are deduplicated, they are
compressed.